The present invention relates to an electric heater for heat treating furnaces, and, particularly, it is suitably used for heat treating equipments to perform, for example, heat treatments such as oxidation, diffusion, and/or CVD of semiconductor wafers.
Conventionally, an electric heater is known which uses a heating element installed at an internal circumferential surface of its cylindrically shaped main thermal insulation body, the heating element being formed to coil shape out of a metal wire referred to as “heavy gauge” whose diameter is 7 to 10 mm.
Then, as a replacement to the above-mentioned electric heater, the present applicant proposed previously an electric heater which is disclosed by a Japanese Patent Application Publication Number 2001-267261. This utilizes a metal wire referred to as “light gauge” of which diameter is 1 to 3 mm, and a plurality of parallel grooves are formed at an internal circumferential surface of its main thermal insulation body extending along its length direction at an interval in its circumferential direction. And a heating element which is made out of a continuous matal wire and formed to sinuous shape with an amplitude bigger than width of the groove is supported unitedly by the main thermal insulation body with both end portions of its amplitude plunging beyond the sidewalls of each corresponding groove into the main thermal insulation body, and meanders in the circumferential direction of the main thermal insulation body so as to extend over all grooves from one to next.
Because a heavy gauge metal wire is used in the aforementioned conventional electric heater, weight of the heating element is heavy, then, it has a large heat capacity. A problem with such a heater is that a fast heat-up or cool-down cannot be achieved. Besides, energy loss per heat cycle is large.
In this regard, the above-mentioned electric heater of the applicant has solved these problems by utilizing a light gauge metal wire.
However, since there is a difference in current specification between the former and the latter types of heater, the latter type heater cannot be employed directly to an existing heat treating equipment where the former type heaters have been installed. This is because wire diameter is different between both types, then, in order that both types of electric heater have an equal output, the latter has to be driven under a high voltage and small current condition, while the former under a low voltage and large current condition. For example, a step-down transformer is required for a low voltage and large current drive, while transformer-less usage is presupposed for a high voltage and small current drive.
Difference in power supply specifications between two types of conventional heater was mentioned above. Then, to enable an utilization of a light gauge electric heater with improved thermal characteristics to replace an existing heat treating equipment where a heavy gauge heater has been used, compatibility in physical or constitutional aspect is also required in addition to its adaptation to power supply specifications. In other words, compatibility related to such as outside diameter, inside diameter, and length of the heater is required, and further, compatibility related to such as division of temperature zones and its power allotment is required to achieve a temperature profile.
Purpose of the present invention is to provide an electric heater which can heat-up and cool-down at a high rate, and moreover, can be driven at a low voltage and large current.